Epoxy resins are preferably used as a matrix resin of a fiber-reinforced composite material obtained by combining an epoxy resin and a reinforcing fiber such as a carbon fiber, a glass fiber, and an aramid fiber, utilizing high mechanical properties, heat resistance, and adhesiveness.
For the manufacture of fiber-reinforced composite materials, a sheet-like intermediate material (a prepreg) in which a reinforcing fiber is impregnated with an epoxy resin is widely used. A molded product is obtained by laminating prepregs and then heating them to cure the epoxy resin. Since various properties can be exhibited according to the laminate design of prepregs, prepregs have been applied to various fields such as aircraft and sports. In recent years, prepregs have also been applied in industrial uses such as uses in automobiles.
A molded product including a prepreg is required to exhibit excellent mechanical properties, and the requirement for the mechanical characteristics, in particular, the elastic modulus of the epoxy resin composition is high. Existing prepregs normally need to be frozen for storage. Thus, a prepreg that is more easily stored and handled is required, and the requirement for storage stability of epoxy resins is also high. In existing prepregs, in particular, in the prepregs of grades that are required to have high heat resistance, 4,4′-diaminodiphenyl sulfone is widely used as a curing agent. A good appearance is required in outer panel uses and the like, and techniques for improving an appearance such as a technique for void reduction have been drawing attention.
Further, though prepregs have been applied to relatively simple shapes in aerospace uses and sports uses, along with the expansion of the application range of composite materials to industrial uses in recent years, efforts to apply prepregs to more complex shapes have been made. However, since the tackiness and drapability required for prepregs are different according to the intended shape, a technique that achieves free control of these properties by adjusting the viscosity and degree of curing of the resin is required. The application to large structural materials such as vehicles and ships has also progressed, and the heat generated in the application needs to be reduced. Accordingly, a B stage technique is drawing attention, in which the viscosity and degree of curing can be controlled by reacting an epoxy resin composition halfway, and instant heat generation can be suppressed due to the multistage curing reaction.
Patent Document 1 discloses an epoxy resin composition that contains 3,3′-diaminodiphenyl sulfone as a curing agent and provides a cured product that exhibits excellent heat resistance and bending modulus. In addition, an epoxy resin composition capable of reducing the amount of heat generation during reaction by the addition of polysulfone as a thermoplastic resin is disclosed.
Patent Document 2 discloses a technique of improving storage stability by adding a sulfonium salt as a catalyst to an epoxy resin composition containing an aromatic amine as a curing agent.
Patent Document 3 discloses a technique of improving storage stability of an epoxy resin composition by using 4,4′-dimethyl-3,3′-diaminodiphenyl sulfone as a curing agent.
Patent Document 4 discloses a technique of introducing holes capable of removing a gas component to laminated prepregs to greatly reduce voids.
Patent Document 5 discloses a technique of introducing an edge breather to remove the trapped gas from the side of the laminated prepreg layer.
Patent Document 6 discloses a technique of continuously controlling the degree of curing by heating during the pultrusion process using curing agents having different reactivity in combination.
Patent Document 7 discloses an epoxy resin composition capable of achieving the B stage state by heating using curing agents or epoxy resins having different exothermic onset temperatures in combination.
Patent Document 8 discloses a technique of increasing the degree of curing of a prepreg by heat-treating a prepreg including an epoxy resin composition containing 3,3′-diaminodiphenyl sulfone and a hydrazine derivative as curing agents in combination.
Non-Patent Document 1 discloses, for an epoxy resin composition containing diaminodiphenyl sulfone as a curing agent, a difference in reactivity with an epoxy resin and a difference in the properties of the epoxy resin cured product obtained by curing the epoxy resin composition depending on a difference in the substitution position of diaminodiphenyl sulfone.